Method of recovering silver



Nov. 2s, 1933.

METHOD OF RECOVERING SILVER Filed July 28, 1933 ATTORNEY4 L. wErsBERG ETAL I 1,937,179

Patented Nov. 28, 1933 UNITED STATES 1,937,179 METHOD 0F RECOVERING SILVER Louis Weisberg and Willard F. Greenwald, New York, N. Y., assignors to Weisberg &'Green wald, Inc., New York, N. Y., a corporation of New York Application July 2s, 1933. serial No. 682,552

12 claim. 101.204,-16)

This invention relates to the recovery of Values from photographic fixing baths and to a method of maintaining such baths at a substantially uniform concentration.

In the fixing of photographic films, sodium thiosulphate, usually called hypo", is employed to remove the unreduced. silver from the-film. The silver thus dissolved concentrates in the fixing bath, and in large industrial operationsin which thousands of feet of film are treated daily, the recovery of the silver is an important factor.

Heretofore silver usually has been precipitated chemically from used hypo solutions by the addition of sodium sulfide in batch operations. The addition of enough sodium sulfide to precipitate all of the silver destroys the value of the hypo solution for further use, and consequently the procedure is uneconomical.

Another suggested procedure involves electrolysis of the used hypo solution under special conditions of current density, temperature and agitation. to deposit silver in metallic form at the cathode. The operation is extremely delicate, especially because of the fact that the current density must be varied with the concentration of silver in the bath, and consequently as the concentration is reduced by deposition of the silver, the current density must be lowered and maintained within extremely narrow limits. The operation, although practical, requires the use of intricate and expensive control mechanism, and is subject to interruption with unsuccessful results if the specified conditions are not continuously maintained.

It is the object of the present invention to provide a simple, inexpensive and effective method of separating by electrolysis a silver compound from used hypo solutions under conditions which are subject to wide Variation without affecting materially the results secured. Another 'object of the invention is the provision 'ofa method of maintaining hypo fixing baths in use at a substantially uniform low concentration of silver, so that the baths are more effective, and the silver content is recovered continuously.

Another object of the invention is the provision of a method of maintaining satisfactorily a substantially uniform temperature in the xing baths, regardless of external conditions of temperature. Among the advantages of the invention, in addition to those hereinbefore mentioned, is an increased recovery of silver owing to the lower concentration maintained in the fixing baths,

since the film traveling through the bath, which carries away a certain amount of solution, re-

moves a lower proportion of silver from a bath of Aaccompanying drawing, which illustrates diagrammatically an apparatus which may be used in carrying out the invention.

While the method is utilized most economically in a system involving circulation of the hypo solution, it may in principle be applied as a batch operation, that is to say, the hypo solution, rich in silver, may be withdrawn from the xing bath from time to time and placed in the electrolytic cell, where the operation is conducted. Such an operation may be used advantageously because the electrolysis of the hypo solution in accordance with the invention removes the silver without impairing the value of the hypo solution for re-use as a fixing bath.

In'carrying out the electrolysis, we may use any suitable tank with insoluble anodes and cathodes. For the cathodes, we may employ any metal which is not attacked by the hypo solution, as for example, stainless steel, silver, or Monel metal. For anodes, we may employ gas carbon, graphite, or a noble metal. Current may be supplied from any suitable source.

The important condition which must be observed in conducting the operation is the use of a current density such as will result in the separation ofthe silver in the form of silver sulfide from the solution. For practical purposes, we

.prefer a cathode current density of approximatethe anode may result in formation of sulphur or rother undesirable oxidation products.

The temperature of the operation is subject to wide variation, but preferably the temperature is maintained at approximately the desirable temperature at which the hypo solution is re-used in the fixing bath (approximately 60-70 F.) Consequently it is desirable to provide cooling and agitating devices in the electrolytic cell. A cooling coil supplied with water or other suitable cooling media may be used. The agitation required is that merely sumcient to maintain proper circulation of the electrolyte and suspension of the silver sulphide as it is separated.

Under the conditions hereinbefore described, silver sulfide separates. at or near the cathode and accumulates in the cell. When the silver content has been reduced to any desirable or predetermined limit, the operation may be stopped. The silver sulfide may then be removed from the hypo solution by decantation, ltration or other convenient operation and the hypo solution is in condition for immediate re-use as a fixing bath. The silver sulde may be subjected then to any suitable operation for the recovery of its silver content.

In the preferred and more economical operation of the method, the electrolysis lis conducted in a substantially continuous operation, the hypo solution being withdrawn continuously from the fixing bath or baths, electrolyzed and recirculated so as to maintain thevlevel in the fixing bath or baths and likewise to maintain a substantially uniform concentration of silver in the fixing solution. This, as already pointed out, facilitates the operation of fixing the film, reduces the loss of silver owing to the drag-out with the film,

. and permits maintenance of the xing baths at a substantially uniform temperature.

In carrying out the invention in its preferred form, we may utilize the apparatus as illustrated in the accompanying drawing, in which 5 and 6 indicate tanks through which the lm is drawn continuously after it has passed through the usual developer and washing tanks. The hypo solution is fed continuously to the tanks 5 and 6 from the return pipe 7 and the branch pipes 8 and 9, controlled by valves 8' and 9'.

The hypo solution is withdrawn continuously through overflow pipes 10 and 11 and delivered to a sump tank 12. From the latter tank the solution is withdrawn through a pipe 13, the flow being controlled by a valve 14 which may be a manually operated valve or may be controlled automatically by a level device attached to the electrolytic cell 15.

In the cell 15, the hypo solution containing silver dissolved from the film is subjected to electrolysis under the conditions hereinbefore described.A Silver sulfide separates from the hypo solution. In the electrolytic cell, the solution may be cooled by a circulating medium in a coil 15. A suitable agitating device 15" may be provided to afford circulation in the cell.

The solution carrying the silver sulde in suspension is delivered by a pipe 16 to a pump 17 and thence through a pipe 18 to a lter or equivalent separating device 19 which removes the silver sulfide and permits the hypo solution freed therefrom to pass through a pipe 20 to a reserve tank 21 which may be provided with an agitator 22 and cooling or heating coils 23. Cooling or heating may be effected by passing any suitable cooling or heating agent through the coils at a rate suitable for the maintenance of any devsird Emperature, preferably that best adapted for the efflcient operation of the fixing baths. v

From the reserve tank 21, the solution is Adelivered by a pipe 24 to a pump 25 which delivers the solution to the return line 7, and thence to the tanks 5 and 6. A pipe 26, connected to the return line, permits the bleeding of any desired portion of the solution in order to avoid excessive accumulation of gelatine in the solution.l

Valves 27 and 28 permit control of the flow of solution to the bleeder and to` the return line.

Additional hypo solution to replenish the supply may be maintained in a tank 29 and delivered to the reserve tank 21 through a pipe 30 controlled by a valve 31. Acid to correct the pH of the solution may be maintained in a tank 32 and supplied through a pipe 33 to the reserve tank 2l. A valve 34 in the pipe 33 is preferably operated by an automatic pH controller which determines the amount of acid fed to the reserve tank.

Due to fluctuations in the amount of film to be fixed and corresponding changes in the rate at which hypo solution is kept flowing through the fixing baths, the level in the electrolytic cell 15 may tend to drop and in order to keep the level of solution in the cell, a by-pass 35 controlled by a valve 36 is provided to permit the return of the hypo solution to the electrolytic cell for recirculation through the pump 17 and filter 19. A valve 37 in the pipe 20 is closed when the valve 36 is open to permit recirculation through the The valves 36 and 37 may also be controlled by a level device in the electrolytic cell.

The level device in the electrolytic cell 15 may be one of a number of Well-known devices adapted for this purpose. Such a level device is adapt- 'ed to be actuated by the rise and fall of the level of -the solution in the electrolytic cell and to close electrical circuits adapted to actuate motors which, in turn, control the valves 14, 36 and 37. In addition to such functions, the level device may control the generator circuit to shut off the supply of current when the level in the electrolytic cell is lowered, and to restore the supply of current when the level again rises to a predetermined point.

When the valve 14 is closed, solution from the tank 12 is permitted to overflow through a pipe 38 to the reserve tank 21, thus permitting a continuous flow of solution circulating from the xing baths to the reserve tank and thence back to the fixing baths, independently of the operation of the electrolytic. cell. Such operation facilitates the maintenance of desired temperatures in the fixing baths, particularly during summer weather when the fixing solution is likely otherwise to attain a relatively high and undesirable temperature.

In the continuous operation of the system as described, the concentration of silver in the fixing baths can be maintained at any desired point (for example not exceeding 1 to 2 grams per liter). In this condition, the hypo solution is most effective as a, fixing agent, and the loss of silver by drag-out is reduced to a minimum.

lli

The fixing baths can be maintained at thedethiosulphate, or any other soluble thiosulphate.

a portion of the solution can be removed from the system and a suitable supply of replenishing solution can be added.

The only control devices ,which need be used in the practice of the invention are the relatively simple level device and the likewise simple mechanism for operating the valves under the control of the level device. Automatic operation by means of the level device is desirable, but the valves may be controlled manually to secure the desired results.

While the invention is particularly applicable to the recovery of silver from fixing baths, it may be utilized in any case where silver is dissolved in a solution of sodium or potassium thiosulphate, as for example in the recovery of silver from scrap film which has not been subjected to a developer solution or in the recovery of silver from any other material from which the silver maybe dissolved in sodium or potassium It is to be understood that the term sodium thiosulphate or hypo as herein used comprehends various formulae including` sodium thiosulphate with other materials such as sodium suIphite, acetic acid, alum, etc., which may be used in combination with sodium thiosulphate in the preparation of a solvent for silver.

Various changes may be made in the details of construction and arrangement of the'apparatus and the procedure without departing from the invention or sacrificing any of its advantages.

' We claim:

1. The method of recovering silver values from solutions containing thiosulphate of an alkali metal, which comprises electrolyzing the solution at a cathode current density in excess of that which will deposit metallic silver at the cathode, whereby the silver separates in the form of noncolloidal silver sulfide, and separating the silver sulde from the solution.

2. The method of recovering metallic silver from solutions. containing thiosulphate of an aikali metal, which comprises electrolyzing the solution at a cathode current density in excess of that which will deposit metallic silver at the cathode, whereby the silver separates in the form,`

of non-colloidal silver sulfide, separating the vsilver sulfide from the solution and treating the silver sulfide to separate metallic silver.

3. The method of recovering silver values from solutions containing thiosulphate of an alkali metal, which comprises electrolyzing the solution at a cathode current density in excess of that which will deposit metallic silver at the cathode, whereby the silver separates in the form of non-colloidal silver sulfide, and filtering the solution to separate the silver sulphide.

4. The method of recovering silver values from solutions containing thiosulphate of an alkali metal which comprises electrolyzing the solution l at a cathode current density of 25 to '75 amperes per square foot, whereby the silver separates in the form of non-colloidal silver sulphide, and separating the silver sulfide precipitate from the solution.

6. The method of recovering silver values from froni'the solution.

solutions containing thiosulphate of an alkali metal which comprises circulating the solution in a cycle, electrolyzing a portion of the solution continuously at a cathode current density in excess of that which will deposit silver at the cathode, whereby the silver separates in the form of non-colloidal silver sulphide separating the precipitated silver 'sulfide from the solution, and returning the solution.

7. The method of recovering silver values from solutions containing thiosulphate of an alkali metal which comprises circulating the solution in a cycle, electrolyzing a portion of the solution continuously at a cathode current density in excess of that which will deposit silver at the cathode, whereby the silver separates in the form of non-colloidal silver sulphide separating the precipitated silver sulde from the solution, modifying the temperature of the solution, and returning the solution.

8. The method of maintaining a photographic fixing bath at a substantially uniform silver concentration which comprises withdrawing continuously a portion of the bath, electrolyzing the solution at a cathode current density at which only non-collaidal silver sulfide isprecipitated, separating the silver sulideand replenishing the fixing bath with the solution from whichv silver sulfide has been separated.

9. rlhe method of maintaining a photographic fixing bath at a substantially uniform silver concentration which comprises withdrawing continuously a portion of the bath, electrolyzing the solution at acathode current density at which only non-collaidal silver sulfide is precipitated, separating the silver sulde, modifying the temperature of the solution, and replenishing the xing bath with the solution from which the silver suliide has been separated.

10. The method of maintaining a photographic fixing bath at a substantially uniform silver concentration which comprises withdrawing continuously a portion of the bath, electrolyzing the solution at a cathode current density at which only non-collaidal silver sulfide is precipitated, separating the silver sulfide, modifying the temperature of the solution, replenishing the fixing bath with the solution from which the silver sulfide has been separated, and by-passing a portion of the solution around the electrolyzing step to maintain the desired temperature in the fixing bath.

11. The method of maintaining a photographic xing bath at substantially uniform silver concentration which comprises withdrawing continuously a portion of the bath, electrolyzing the solution withdrawn at a cathode current density of 25 to '75 amperes per square foot, whereby the silver separates in the Aform of non-colloidal silveil sulphide separating the precipitated silver sulfide and returning the solution for further use in the fixing bath.

12. The method of maintaining a photographic fixing bath at substantially uniform silver concentration which comprises withdrawing continuously a portion of the bath, electrolyzing the solution withdrawn at a cathode current density of 25 to '15 amperes per square foot, whereby the silver separates in the form of non-colloidal silver r sulphide separating the precipitated silver sulfide from the solution, modifying the temperature of the solution, and returning the solution for further use in the fixing bath.

` LOUIS WEISBERG.

WILLARD F. GREENWALD. 

